The assays being developed by researchers in the genomics, proteomics, ADME/TOX and other markets are moving to higher throughputs with smaller liquid volumes. To address this change, the suppliers of plastic labware have developed multiple well plates. The multiple well plates have steadily increased the number of sample wells from 6 to 12, 24, 96, 384, 1536 and higher densities are expected in the near future.
The filter bottom multiple well plates have lagged behind in array densities because of manufacturing complexities. To further complicate the filter plate manufacturing are the increased performance attributes required in a filter plate. A typical assay run on a solid plastic bottom multiple well plates consists of adding and removing liquids from the opening of the wells with a pipettor, or automated liquid handlers.
The sample preparation methods developed for use with multiple well filter plates typically either filter the sample or concentrate the sample. An example of a method that concentrates the sample would be a bind and elute method, where the filter matrix provides specific binding of a species in the sample solution. When the sample is passed through the filter the species is bound to the matrix and the remaining solution passes through the filter unaltered. A second solution is passed through the filter that elutes the species. This elution solution needs to be collected. Typical devices for this method have an underdrain and spout downstream of the filter to collect the filtrate and direct it to the appropriate well in a collection plate.
An example of a multiple well filter bottom plate being used to filter a sample would be a lysate clearing application. The researcher would lyse a batch of cells, thereby releasing the intercellular matter within the cells. A filter is used to stop the large debris, like the cell walls, while letting components like the DNA to pass freely through the filter. Similarly, the filtered solution is collected into a collection plate by means of an underdrain and a spout.
An example of a product suitable for these two applications is the Multiscreen™ plate, which is manufactured by Millipore Corporation of Bedford, Mass. and is disclosed in U.S. Pat. No. 4,902,481.
Another method of concentrating components in a solution is by using an ultrafiltration filter. When concentrating with an ultrafiltration filter the components of interest are retained or concentrated by the ultrafiltration filter. Typical concentrating devices have been limited to single sample processing devices such as a Centricon® or Microcon® manufactured by Millipore Corporation of Bedford, Mass. which have a single well in the form of a large tube, similar to a test tube and a filter sealed along the side or bottom of the well. One example of a multiple well ultrafiltration device for concentrating a sample is disclosed in U.S. Pat. No. 5,674,395. In this patent, one or more filter panels are secured to the sidewalls of the filter device.
The construction of an ultrafiltration filter has a thin layer of polymer forming an ultrafiltration skin matrix. Typically, the skin matrix is formed on top of a more open supporting structure. This basic construction makes it difficult to seal into plastic devices. The sealing methods used to fabricate plastic devices containing ultrafiltration filters can be put into two categories: seal to the skin matrix or seal through the support structure.
When sealing through the support structure, it is important that the support structure is sealed off from liquid flow. If the support structure is not sealed then the sample will preferentially pass through the support structure because of its open structure relative to the skin matrix, and if that happens the fluid will not be filtered. One sealing method used to seal to the support structure is with heat to collapse the support structure or to fill it with molten plastic from the part to which it is bonded, so that the only filtering path is through the ultrafiltration skin matrix.
When sealing to the skin matrix, gaskets, such as o-rings and elastomeric seals, have provided the most utility in making an integral seal between the housing and skin matrix of the ultrafiltration filter. It has been possible to seal to the skin matrix using adhesives in spiral wound ultrafiltration cartridges, and also a single sample processing centrifuge device described in U.S. Pat. No. 5,647,990. However, this application has not been useful in expanding the role of adhesives for use with UF filters in multiple well plates.
When filtering samples in multiple well array, the cross contamination of samples due to leaks between wells can lead to unacceptable results. The Multiscreen™ product has overcome this problem by isolation of the individual filter in each well. This is typically accomplished by insertion and sealing of individual filters into each well. This can be a time consuming and labor intensive operation especially as one goes to smaller and smaller well diameters (e.g. 384 well plates, etc). Also in those smaller well diameter applications, the ability to effectively seal the individual filters in each well becomes more complicated and difficult. If collection of the filtrate from each well is desired, an underdrain is provided to collect and deliver the filtrate from one well to a corresponding collection well. The underdrain must seal to the downstream side of the filter or plate, which further complicates the manufacturing of such a product.
In some applications, such as cleaning up a PCR product, it is possible to use the size exclusion properties of an ultrafiltration filter. Selecting the appropriate pore size filter and then do repeated washing through the filter to remove the salts and other small impurities, then collect the cleaned PCR product off the filter surface. In this approach the need for individual filters has been eliminated because the sample will be retained on the membrane surface and the smaller unwanted materials will pass through the filter. The limitation to this approach is the entire plate needs to be run and the liquid flowing through the filter should be drawn away to eliminate back migration of small components such as salts from the support structure. The running of an entire plate in the high throughput laboratory is common, actually it would be rare to have applications using partial plates.
It would be desirable to make a multiple well filter plate with an ultrafiltration filter for concentrating samples that does not suffer from crosstalk and is compatible with the methods and equipment used in the high throughput laboratories.
It would be further desirable to make the multiple well plate with one common sheet of ultrafiltration filter so as not to have to cut and position individual filter panels for each sample well.
It would be further desirable to make a multiple well plate with a common sheet of microporous filter suitable for size exclusion concentrating applications.